A multiplexer or demultiplexer enables you to expand the in-and outputs on your Arduino board.

The 4051 is an 8 channel analog multiplexer / demultiplexer:

• If you use the 4051 as a Multiplexer: You can choose between 8 different inputs and select just one you want to read at the time.
• If you use the 4051 as a Demultiplexer: You can choose between 8 different outputs and select just one you want to write at the time.

Futhermore, the 4051 is able to work with analog values; in the case of the Arduino, you are able to use the analog inputs with a voltage between 0-5V and route them to an Analog-In Pin on your Arduino.

TUTORIAL FOR ARDUINO

Arduino Library